1. Field of the Invention
The present invention relates to surface acoustic wave (SAW) devices and, in particular, to the use of polyimide absorbent layers patterned on the surface of a SAW device for the purpose of damping spurious signals. 2. Discussion of the Prior Art
Surface acoustic wave (SAW) devices of the type that utilize a piezoelectric substrate for propagating surface waves between a transmitting transducer and a receiving transducer are well known. Typically, the interdigital transducers, or IDTs, are formed as arrays of interleaved aluminum electrodes deposited on the surface of the substrate. In response to electrical input signals, the transmitting transducer launches corresponding acoustic surface waves along a predetermined path on the surface of the substrate. The receiving transducer detects the acoustic waves and generates corresponding electrical output signals.
In SAW devices of the type described above, the transmitting transducer launches surface waves in opposite directions simultaneously. Similarly, the receiving transducer receives surface waves from either direction. This results in a significant problem in most SAW devices because, in addition to the receiving transducer responding to surface waves travelling directly from the transmitting transducer to the receiving transducer, both transducers respond to spurious surface waves which have been reflected from the edges of the substrate. These reflected surface waves produce unwanted signals that distort the main, desired signal, adversely affecting the performance of the SAW device.
Various methods have been proposed for suppressing edge-reflected surface waves. The most common approach is to apply an acoustic absorber on the substrate surface between the edge of the substrate and the back edge of the adjacent transducer. The geometry of the absorber is chosen so as to be sufficient to absorb the unwanted surface waves launched toward the edges of the substrate and reflected back to the IDTs from the edges. Commonly-used absorbent materials include RTV (room-temperature vulcanizing silicone rubber), black wax and epoxy resin.
An example of the use of an acoustic absorbent material for suppressing spurious surface waves is disclosed in U.S. Pat. No. 4,354,129 titled "Absorber For Piezoelectric Surface Acoustic Wave Device", issued to Ieki on Oct. 12, 1982. Ieki teaches a SAW device which includes a lead-zirconium-titanate (PZT) substrate, transmitting and receiving transducers mounted on the surface of the substrate for propagating acoustic waves between the transducers, and epoxy resin acoustic absorbers formed on the substrate between the end of the substrate and an adjacent transducer for absorbing unwanted acoustic waves. The shape of Ieki's absorber is related to the energy distribution of the acoustic waves. The epoxy absorbers disclosed by Ieki are deposited on the SAW device substrate by conventional techniques such as screen printing.
A second example of the use of acoustic absorbent materials on SAW devices is provided by U.S. Pat. No. 4,516,095 titled "Surface Acoustic Wave Device", issued to Lee on May 7, 1985. Lee teaches a SAW device having acoustic absorbers positioned both at the edges of the substrate as well as between the transmitting transducer and the receiving transducer. The acoustic absorber located between the transducers increases the insertion loss of the device and, consequently, suppresses "triple transit signals" which result from the interaction between the input and output transducers. Lee discloses that the absorbent material used both between the transducers and at the edges of the substrate may be chosen from among silicone rubber, epoxy resin, a mixture of silicone rubber and epoxy resin, each of the foregoing mixed with an oxide powder, and wax. According to Lee, the acoustic absorbent material is applied to the SAW device substrate by conventional screen printing techniques.
The above-described approach to suppressing spurious edge-reflected acoustic waves suffers from a number of major disadvantages. First, the techniques used for applying the absorbent material, i.e. either painting or screen printing, are either extremely labor intensive or very expensive, involving either manual or robotic application of the absorbent material to individual SAW devices. Second, both the type of absorbent material used and the technique for applying it require high dimensional tolerances to avoid interference with the adjacent transducers; this results in unnecessarily large SAW devices. Third, since the absorbent material is applied by relatively crude techniques, the volume of material applied and its resulting geometry is poorly controlled, often resulting in undesired wave reflection from the absorbent material itself. Fourth, all of the commonly-used absorbent materials are unstable at higher temperatures, limiting both subsequent processing techniques for fabricating SAW devices as well as the applications in which the completed SAW devices can be utilized.
U.S Pat. No. 4,642,507 titled "SAW Devices With Reflection-Suppressing Fingers", issued to Suthers et al on Feb. 10, 1987 and European patent Application No. 86301324.9 titled "SAW devices including resistive films", filed by Oliver et al and published on Oct. 12, 1986 disclose the utilization of a silicon film acoustic wave absorber in conjunction with a SAW device. However, for this application, silicon absorbers can only be successfully applied to substrates having high piezoelectric coupling.
It would, therefore, be highly desirable to have available a simple, inexpensive technique for the precision application of a temperature-resistent, acoustic wave absorber material to a wide range of SAW device substrates for the purpose of damping spurious signals.